1. Field of the Invention
The present invention relates to an exhaust gas heat exchanger for performing heat exchange between exhaust gas generated by combustion and cooling water. Specifically, the present invention relates to an exhaust gas heat exchanger for cooling the exhaust gas in an exhaust gas recirculation system (i.e., EGR system).
2. Related Art
As shown in FIGS. 1A and 1B, as a prototype made by the inventors, an exhaust gas heat exchanger for cooling the exhaust gas in an EGR system (hereinafter, referred to as an EGR gas heat exchanger 300) can be equipped with plural laminated exhaust gas tubes 301 disposed in a tank 302 having a rectangular sectional pipe shape. The exhaust gas tubes 301 have a flat sectional shape, and are attached to a core plate 303 which closes the tank 302. A cooling water inlet pipe 304 and a cooling water outlet pipe 305 are connected to the tank 302 so that cooling water flows in the tank 302 to exchange heat with the exhaust gas passing through the exhaust gas tubes 301.
In this prototype, the inventors have found that the cooling water might be boiled at a location close to an upstream side of the exhaust gas tubes 301. The boiling of the cooling water may cause less efficiency about cooling of the exhaust gas flowing through the exhaust gas tubes 301, and/or rapid increase of inner pressure of the tank 302 that may degrade durability of the tank 302.
The inventors performed an experiment to visually observe the stream of the cooling water flowing in an EGR gas heat exchanger that has four exhaust gas tubes.
According to this experiment, when the cooling water inlet pipe is connected to the tank 302 so as to be disposed substantially perpendicular to a longitudinal direction of the exhaust gas tubes 301, the cooling water flows into each passage formed between each adjacent exhaust gas tubes 301 so as to turn approximately perpendicular as shown by arrows A (cooling water stream A) in FIG. 2, and it flows toward the cooling water outlet pipe 305. Moreover, some of the cooling water collides (impacts) with an inner wall 302a of the tank 302 that is opposite to the cooling water inlet pipe 304 as shown by arrows B (cooling water stream B) in FIG. 2, and then, it flows toward an exhaust gas pipe 301 located at an outermost side.
However, the cooling water stream A coming from the cooling water inlet pipe 304 and the cooling water stream B coming through the passages formed between each adjacent exhaust gas tubes 301 interfere with each other at the gaps formed between the inner walls 302a and the outermost exhaust gas pipes 301. As a result, the cooling water is easily stuck in the vicinity of the root portions of the exhaust gas pipes 301 where the exhaust gas pipes 301 are fixed to the core plate 303, as shown in FIGS. 1 and 3.
This means it may be possible to boil the cooling water when the cooling water is stuck in the vicinity of root portions of the exhaust gas pipes 301 of an upstream side of the exhaust gas. As a result, the efficiency for exchanging heat may be lowered.
Moreover, the local boiling of water may be caused by low flowing rate of the cooling water in the tank 302.